The present invention relates to a semiconductor device including an optical semiconductor element for emitting light of a wavelength ranging from a blue region to a ultraviolet region.
In a highly developed information society where image information with high capacity is transported through cellular phones or internet, there is a demand for development of optical disk technique with higher speed and higher capacity for storing the high capacity information. In order to cope with such a demand, a semiconductor laser diode used in an optical pickup of an optical disk apparatus has been reduced in the wavelength from 780 nm to 650 nm so as to increase the capacity of an optical disk, and the wavelength is being further reduced to a 400 nm band for further increasing the capacity of an optical disk.
Also, an optical semiconductor element such as a semiconductor laser diode for emitting light of a wavelength of a 400 nm band or in the vicinity of 400 nm, that is, a region of a blue, violet or ultraviolet, is built on a package including a metal base, so as to be fabricated as a semiconductor device in which a gas is encapsulated within a space surrounded by the package and a cap having an optical member for taking out emitted light.
In the semiconductor device thus fabricated, various substances may adhere within or enter the space enclosed by the package including the optical semiconductor element and the cap during the fabrication. When light emitted by the optical semiconductor element outgoes from the end facet of a cavity with the various substances present within the enclosed space, a product is produced and adhered onto the end facet of the cavity, and therefore, the optical characteristic and the electric characteristic of the optical semiconductor element of the semiconductor device are disadvantageously degraded.
In order to prevent the thus produced product from adhering as a contaminant onto the emitting end facet of the optical semiconductor element, a structure in which an absorbent is provided within the space where the optical semiconductor element is encapsulated has been disclosed (see, for example, Japanese Laid-Open Patent Publication No. 2004-14820). This absorbent prevents the product from adhering onto the emitting end facet by previously absorbing the product and a material of the product floating within the space where the optical semiconductor element is encapsulated. However, since the absorbent is disposed in the vicinity of the optical semiconductor element, the semiconductor device including them is increased in the size, and hence, whole equipment including the semiconductor device cannot be made compact and thin.
Therefore, in order to solve the aforementioned problem with the general package shape retained and without increasing the size of the semiconductor device, a structure in which the product is prevented from adhering onto the end facet of the cavity by suppressing low the dew-point and the oxygen concentration of the atmosphere of the space where the optical semiconductor element is encapsulated has been proposed. Specifically, the production of the product that may black the end facet of the cavity with time is thus prevented, and the lifetime of a nitride semiconductor laser diode for emitting light of a wavelength not more than 420 nm is increased (see, for example, Japanese Laid-Open Patent Publication No. 2006-13436).
Furthermore, as a method for suppressing low the dew-point and the oxygen concentration of the atmosphere within the space where the optical semiconductor element is encapsulated, a fabrication method additionally including a baking process for cleaning by previously baking a semiconductor laser diode, a cap and a package or including an ashing process for cleaning by exposing the semiconductor laser diode and the like to ozone and excited oxygen atoms has been proposed (see, for example, Japanese Laid-Open Patent Publication Nos. 2006-128629 and 2006-140441). When such a process is additionally performed, a nitride semiconductor laser diode can be driven for light emission for a long period of time without lowering the emission intensity even in a high temperature atmosphere.
However, in the structures or methods disclosed in Japanese Laid-Open Patent Publication Nos. 2006-13436, 2006-128629 and 2006-140441, the product cannot be prevented from adhering onto the emitting facet of the optical semiconductor element unless the dew-point is suppressed as low as −30° C. or less and the oxygen concentration is suppressed as low as 100 ppm or less in the atmosphere of the space where the optical semiconductor element is encapsulated. Furthermore, the semiconductor device is difficult to operate for a long period of time without lowering the optical output from the end facet of the optical semiconductor element unless the dew-point and the oxygen concentration are suppressed to sufficiently low values in the enclosed space. Moreover, in order to attain a dew-point of −30° C. or less and the oxygen concentration of 100 pm or less in the atmosphere of the space where the optical semiconductor element is encapsulated, it is necessary to fabricate the semiconductor device by employing special facilities, for example, by providing an apparatus used for assembling the optical semiconductor element within a glove box. Thus, it is impossible to fabricate such a semiconductor device at low cost with simple facilities.